1. Field of the Invention
This invention relates to a surface type acceleration sensor and a manufacturing method thereof which is manufactured when a thin film formed on the surface of a silicon base plate is subjected to etching.
2. Background
A strain gauge type acceleration sensor 40 of the bulk type shown in FIG. 23 is well known as an acceleration sensor used for ABS (the anti-lock brake system), the air bag system or the suspension control system of an automobile.
This type acceleration sensor 40 is manufactured in such a manner that a bulk of the single crystal base plate 41 made of silicon, the shape of which is rectangular parallelepipedic the crystal plane index of which is (100), is selectively etched from the front and reverse sides, that is, a bulk of the single crystal base plate 41 is subjected to crystal anisotropy etching. A cantilever structure portion 42 formed by etching includes a cantilever 43 and a mass portion 44. The mass portion 44 functions as a weight and is disposed at an end of the cantilever 43. A plurality of strain gauges 45 are provided onto an upper face of the cantilever 43. When an acceleration is impressed upon this acceleration sensor 40, the mass portion 44 is displaced in a predetermined direction, so that the cantilever 43 is bent. At this time, strain is induced in the strain gauges 45 provided on the upper face of the cantilever 43. As a result, by the piezo-resistance effect of silicon, resistance values of the strain gauges 45 are increased or decreased. When the changes in these resistance values are detected, the acceleration can be found.
In this connection, in order to obtain a predetermined detection sensitivity by the conventional acceleration sensor 40, it is necessary to use a single crystal base plate 41 made of silicon, the thickness of which is at least 200 to 300 .mu.m so that a thick mass portion 44 can be formed.
However, when the thick single crystal base plate 41 of silicon is etched from the reverse side, consideration must be given to the anisotropic etching characteristic, that is, consideration must be given to that an etching hole, the opening angle of which is .theta.=125.26.degree., is formed along the plane of (111). Therefore, it is necessary to set the size of an opening portion "a" at a fairly high value. Due to the foregoing, width W of the chip is increased. Therefore, it is difficult to sufficiently reduce the dimensions of the overall acceleration sensor 40.
As shown in FIG. 24, the bulk type acceleration sensor 40 is commonly joined onto another base plate 47 via a die bonding agent 46 coated on the reverse side of the single crystal base plate 41 of silicon. In this case, it is necessary to avoid the deposition of the die bonding agent 46 onto the mass portion 44, and it is also necessary to avoid the contact of the mass portion 44 with the base plate 47 so that the mass portion 44 can be freely displaced. However, in order to accomplished the above object, it is necessary to arrange a mount 48 on the reverse side of the single crystal base plate 41 of silicon. Therefore, the packaging work of the acceleration sensor 40 becomes complicated.
As a different type sensor from the above bulk type acceleration sensor 40, there is provided a surface type acceleration sensor which is manufactured when a thin film formed on the front side of a silicon base plate is etched. For example, Japanese Examined Patent Publication No. 4-71344 discloses a method of manufacturing the above type acceleration sensor by "anode formation". The summary of the method will be briefly described as follows.
First, a portion of the p-type single crystal silicon base plate is subjected to anode formation so as to form a porous silicon layer. Then, a p-type single crystal silicon layer is epitaxially grown on the surface. Next, a portion of the epitaxial growth layer is removed, and a porous silicon layer exposed from the removal portion is oxidized. Then, an N-type diffusion strain gauge is provided at a predetermined position on an upper face of the epitaxial growth layer. Then the oxidized porous silicon layer is etched by hydrofluoric acid, so that a cavity is formed in a lower portion of the epitaxial growth layer. Finally, an electrode is formed in the diffusion strain gauge. In this way, manufacture of the surface type acceleration sensor is completed.
However, the following problems (1) to (4) may be encountered in the above manufacturing method.
(1) According to the above method, a mask of Si.sub.3 N.sub.4 is disposed on the p-type single crystal silicon base plate, and the opening portion is subjected to anode formation. Accordingly, the dimensions of the anode formation portion tend to fluctuate. Therefore, it is necessary to severely set the processing temperature and the processing time.
(2) It is very difficult to form an epitaxial growth layer on the porous silicon layer.
(3) Since the epitaxial growth layer is of the p-type, it is necessary to use a diffusion strain gauge of the n-type. Compared with a case in which the p-type diffusion strain gauge is used, the gauge factor is reduced in this case. Therefore, it is impossible to obtain a predetermined detection sensitivity.
(4) After the anode formation portion has been etched, a wiring pattern is formed. Therefore, the resist enters a cavity in the process of photolithography. For this reason, it is necessary to remove the resist that has entered.
In the case of the acceleration sensor described in Japanese Examined Patent Publication No. 4-71344, it is difficult to form a large mass portion. Difficulty of forming a large mass portion is not limited to the above case. In general, forming a large mass portion is difficult in the case of a surface type acceleration sensor. Therefore, advent of a surface type acceleration sensor having the same detection sensitivity as that of the bulk type acceleration sensor has long been desired.